1. Field of the Invention
This invention relates to shot peening and, more particularly, to measuring shot peening intensity with Almen test strips.
2. Description of Related Art
Shot peening has been common practice in the treatment of metal components to increase or restore fatigue life. Spherical shot is impacted on the surface of a component forming very small spherical dents on the surface and imparting compressive residual stress in the component in the shot peened surface of the component. It is highly desirable if not required to control intensity of the shot peening because intensity above and below a critical intensity range can result in a component having less than optimal fatigue life properties.
Shot is typically accelerated by using air pressure to force the shot through a peening nozzle which is directed at the surface undergoing peening. Almen strips are used to measure shot peening intensity by their curvature which is typically referred to as arc height. Note that arc height or curvature is sometimes referred to as deflection. Almen strips are precision (hardness and thickness controlled) thin 1070 steel strips. Each strip is bolted to a holder which restrains the strip from curving until the bolts are released. The strip is then exposed to the shot stream under the same conditions as the component undergoing peening. After the strip has undergone peening for a predetermined time period, the strip is removed from the holder and the arc height (curvature) measured, all according to specification prescribed procedures, using an Almen gage containing a dial (or digital) indicator or gage. Typically, a scrap part is appended with a number of Almen strip holders so that intensity may be determined in a number of locations.
Accordingly, a series of Almen strips are exposed to the shot stream for increasing time periods at each of these locations. When the arc height of each of the Almen strips increases by no more than ten percent (10%) when the time is doubled, the arc height is declared to be the intensity and peening of the component parts may begin at this intensity if all are within the required range. Intensity is expressed in terms of measurement of the arc height, e.g. inches.
Intensities in less accessible areas, where holders cannot fit, can sometimes be inferred by similarity in impingement angle to Almen strip equipped areas. In critical life areas inference is often not enough. In areas that are smaller than the Almen strip or accessible only by ricochet, a better method is highly desirable. One method uses “shaded” strips in which a full strip is mounted in a fixture exposing only a stripe representing the size, location and accessibility issues of the part area to be shot peened. The use of “shaded” strips in small areas involves an expensive tooling cost to insert the shaded strip holder into a fixture simulating the part to be peened. The use of “shaded” strips requires an added step in the peening process setup because an additional setup and peening test must be conducted to correlate the shaded strip arc height to full strip values which must be within the required intensity range. A more accurate, less costly, and less time consuming shot peening intensity measuring method is highly desirable.